Oxygen analyzer



April 25, 1967 G. BERGSON ETAL 3,316,166

OXYGEN ANALYZ ER 1N VEN TORS GUSTAV BERGSON 8 PETER KETELSEN April 25,1957 G. BERGSON ETAL. 3,316,166

OXYGEN ANALYZER Filed May 15, 1965 3 Sheets$heet 2 ii l: Il Il' GUSTAVBEHGS/V PETER KETELSE/V April 25, 1967 G. BERGSON ETAL OXYGEN ANALYZER 3Sheets-Sheet 3 Filed May 15, 1963 www...

wN www w WSL m NGF. 0 Im- M 5M V mm Z ww GP United States Patent Oiitice3,315,16@ Patented Apr. 25, 1967 This invention relates to gas analyzingsystems, and more particularly to improved systems for the electrolyticmeasurement of the oxygen concentration of a gas stream.

Electrolytic oxygen analyzers have heretofore been proposed which dependfor operation on the quantitative reduction of gaseous oxygen with theconcurrent liberation of electrons which constitute a flow of current,the magnitude of which is related to the oxygen concentra-y tion inaccordance with Faredays Law of Electrolysis. Brieliy, a gas stream tobe analyzed is passed at a known rate of flow through a cathode which isin contact with an aqueous electrolyte solution. The cell is operated atpotentials which do not cause dissociation of the electrolyte whilemaintaining the potential of the cathode with respect to the electrolytesolution suliciently negative, with respect to that which wouldcorrespond to equilibrium, to insure essentially quantitative operationwhere in each atom of the oxygen molecule gives up two electrons whilegoing over into the hydroxyl ion at the cathode. In similar' fashion,the principal electrolysis anode is maintained at a potentialsufficiently positive with respect to that which would determineequilibrium to reverse the cathode action. Experience shows that thesepotentials can be maintained without violating the requirement withregard to the cell electrolyte.

The anode and cathode elements must be of a type which are not subjectto rapid attack by the electrolyte solution. For example, with apotassium hydroxide (KOH) solution, the cathode material may comprisesilver or copper, and the principal anode may comprise a stainless steelcontainer for the cell. A third electrode included in known quantitativeoxygen analyzers consists of a large rod which may be cadmium. Thehydroxyl ions formed at the cathode are released as molecular oxygen atthe stainless steel anode, and are carried out of the system by the gasstream bubbling through the electrolyte. Qualitative oxygen analyzersystems heretofore available have been bulky and expensive inconstruction. One reason is that the known oxygen analyzer cells per seare quite large and diiiicult to build. Another reason is that the gasflow regulating apparatus of prior systems must be constructed ofmaterials which resist attack by the corrosive electrolyte solutionpicked up in vapor and molecular form by the gas stream.

As a result of their resultant size and Weight, existing oxygenanalyzing systems are considered to be semipermanent installations, andare not Well adapted for use in applications requiring a portableinstrument. In addition to the foregoing problems with respect to size,weight and expense, it has -been found that the hold up time of existingapparatus is sometimes undesirably long. The hold up time of theinstrument is the time it takes the instrument to indicate changes inthe oxygen content of a gas stream from the time the gas stream With the-changed oxygen content is introduced into the system.

It is an object of this invention to provide an improved oxygenanalyzer.

A further object of this invention is to provide an improved oxygenanalyzer which is relatively small, light in weight and inexpensive ascompared to known types of oxygen analyzers, so as to be useful as aportable apparatus.

Another object of this invention proved oxygen analyzer cell.

Still another object of this invention improved is to provide an irngenanalyzers. n An oxygen analyzing system embodying the invention includesa gas inlet port for admission of the gas stream oxygen analyzer cellinto the wetter cell and, subsequently, the gas inlet lines.

After the gas has passed through the l oxygen analyzer cell, it ispassed through a neutralizer.

The gas outlet contact area for the gas stream as it passestherethrough. it has been found that Virtually all of the KOH inmolecular form reacts with the aluminum and is hence removed from thegas stream.

As a consequence of removing the corrosive electro* lyte material fromthe gas stream, the gas flow rate conpressure regulator, valves,rotometers etc., may comprise items which are relatively inexpensive ascompared to components which must be built in a manner to resist thecorrosive effects of the electrolyte. Thus, the neutralizer cellcontributes substantially to the reduction in cost of the oxygenanalyzer system of the invention as compared to prior systems.

Another aspect of the invention pertains to the oxygen The oxygenanalyzer housed in a light Weight transparent tubular casing of luciteor the like, and includes a porous active cathode, and three anodeelectrodes. The cathode, Which may, for example be comprised of poroussilver, is mounted near the end of the casing which comprises the gasinlet port. A first anode which extends to a position in the vicinity ofthe cathode provides a dual function of shielding the cathode fromimpurities and establishing the solution potential. The first anode may,for example, comprise a loop of cadmium which has a substantial surfacearea confronting the cathode. A third anode electrode is: provided forcompleting the reaction process. The third or principal anode comprisesa strip of material such as stainless steel, and has a surface area ofthe order of that of the cathode.

A second anode electrode included in the tube comprises an impuritycollector. As described in Flock et al. 2,898,282, minor amounts ofreducible impurities are contained in the electrolyte solution some ofwhich result from the gradual corrosive action of the electrolyte on thethird anode electrode. These impurities tend to coat the first anodecausing ultimate cell failure. In prior cells this electrode was maderelatively large to delay such failure at the expense of size andweight. This problem is reduced in the cell of the invention by thesecond electrode which is maintained at a potential to attract theimpurity ions and thereby prevent undesirable coating of the rst anode.Since the problems caused by the impurity coating is substantiallyobviated by the second anode operation, the rst anode can be maderelatively small in size, thus substantially reducing the size, weightand cost of the analyzer cell. The resultant smaller size of theanalyzer cell permits the practical use of transparent, easilymachinable materials as a container for the electrolyte and electrodes,thereby permitting instantaneous operational checks on the state of themeasuring equipment as it relates to proper maintenance of replaceablematerials. In addition, the smaller analyzer cell Ipermits shorterlengths of tubing to 'be used in c-onnecting the cell with the remainderof the system, thus further contributing to overall system sizereduction.

An appropriate electrolyte, such as a KOI-I solution, is introduced intothe cell, and appropriate potentials are applied to the electrodes aswill hereinafter be described.

Another important aspect of the invention comprises the novel wettercell through which the gas passes prior to its admission to the analyzercell. The purpose of the wetter cell is to establish the moisturecontent of the gas stream at an equilibrium condition for thetemperature of the electrolyte solution in the analyzer cell, Thus, thegas stream in passing through the electrolyte gives up as much moistureas it absorbs, and hence does not tend to remove moisture from theelectrolyte. The wetter cell of the invention comprises a porous ceramicor like enclosure immersed in an aqueous solution in a manner that thesolution can get into the enclosure only by diffusion through the porouswalls thereof. The gas stream is ad mittedinto the enclosure, and picksup moisture by circulating in contact with the wet inner walls thereof.This Wetter cell construction keeps the hold-up time of the analyzingsystem short because the gas stream does not bubble through thesolution, and hence oxygen in the gas stream is not picked up by thesolution. Thus if the oxygen content in the stream is changed, the timerequired to establish an equilibrium condition due to oxygen beingabsorbed or given up by the solution is very short.

To prevent the solution from collecting inside the enclosure, thediffusion rate of the solution through the porous material may becontrolled by coating a portion of the solution contacting surface ofthe porous enclosure with a non-porous blocking material. As will bedescribed more fully hereinafter, the rate of diffusion of theA solutionthrough the porous material is automatically controlled as a function ofthe amount of moisture picked up by the gas stream with the rate beingsuch as to prevent accumulation of the solution in the interior of theenclosure.

The novel features which are considered to be characteristic of thisinvention as well as additional objects and advantages thereof will beunderstood more clearly when read in connection with the accompanyingdrawings in which:

FIGURE 1 is a diagrammatic and block diagram oxygen analyzing systemembodying the invention;

FIGURE 2 is an elevational view, in section, of a wetter cell embodyingone aspect of the invention;

FIGURE 3 is a sectional view of the wetter cell taken on section lines3-3 of FIGURE 2;

FIGURE 4 is an elevational view, in section, of an oxygen analyzer cellembodying another aspect of the invention;

FIGURE 5 is a top view of the oxygen analyzer cell shown in FIGURE 4;and

FIGURE `6 is a schematic circuit diagram of the power supply andmetering circuits for connection to the oxygen analyzer cell.

of an Referring to FIGURE l, the Ioxygen, analyzing system of theinvention comprises an inlet port 10 to which gas under pressure isadmitted. The inlet port Il) is connected to the gas outlet port 12 by abypass valve 14 and a bypass rotometer 16. Gas from the inlet port 10 isdirected through a wetter cell 18 which is mounted on a bracket 19affixed to a chassis Ztl for the apparatus. From the wetter cell 18, thegas is passed through a tube 22 to a trap 24 which comprises a tube oftransparent material of sufficient volume to prevent back-flow of fluidsinto the wetter cell 1S. The outlet side of the trap 24 is coupled by apipe 26 to an oxygen analyzer or electrolytic cell 28.

The analyzer cell 28 includes a cathode mounting block and terminal 36,and first, second and third anode mounting blocks and terminals 32, 34and 36 respectively. The respective mounting blocks and terminals areconnected to a power supply, amplifier and metering circuit 38.

After the gas has passed through the analyzer cell 28 it is directedthrough a tube lil to a neutralizer cell 42. The neutralizer cell isfilled with a material which reacts with the electrolyte solution, sothat any of the electrolyte solution picked up by the gas stream isremoved before the gas leaves the neutralizer cell. In the present case,where the electrolyte used in the analyzer cell 28 is KOH, theneutralizer cell is filled with aluminum pellets which provide a largesurface area to the gas passing therethrough. The neutralizer cellhousing comprises a transparent tube of lucite or the like so the visualcondition of the neutralizer material may be easily ascertained by theoperator of the apparatus.

The gas ow rate through the analyzer cell 28 is controlled yby adifferential pressure regulator 44 and a valve 46, connected in series,to the outlet port of the neutralizer cell 4t2. The gas flow rate isindicated by a rotometer 48 connected in the gas line between the valve46 and the outlet port 12.

The trap 24, analyzer cell 28 and neutralizer cell 42 are mounted on theapparatus chassis 2t] by suitable mounting means indicated in FIGURE lby the blocks 5l?. The lower end of the analyzer cell is preferably wellbelow that of the wetter cell 18 to minimize the risk of back-flow ofthe electrolyte from the analyzer cell 2S when the inlet port 10`pressure is removed. Any back pressure tending to produce electrolyteback-flow may be further dissipated by using capillary tubing Vfor thetubes 22 and 26.

The wetter cell 18 which is shown in greater detail in FIGURES 2 and 3,comprises a transparent tubular casing 52 clamped between a pair of endplates 54 and 56 by a plurality of bolts 58. Suitable gaskets 60 areprovided between the casing 52 and the end plates to provide a watertight enclosure. A porous ceramic cup 62 is positioned within the watertight enclosure, and clamped between a clamp plate 64 and the bottomplate 56 by a plurality of bolts 66. A gasket 68 is provided at thelower end of the cup 62 to provide a further water tight enclosurewithin the cup. A gasket 70 is provided between the top of the cup 62and the clamp plate 64 to prevent damage to the cup when the bolts 66are tightened.

A gas inlet opening 72 for yconnection with the inlet port 10, and a gasoutlet opening 74 for connection with the pipe 22 (FIGURE l) areprovided in the bottom plate 56. A tube 73 conveys the gas admitted tothe wetter cell well up in the cup 62 to insure that the gas circulatesagainst the wet walls of the cup. The top plate includes a threadedopening for receiving a fill plug 76. Water or other suitable solutionis admitted to the wetter through this opening. A second threadedopening 'i8 is provided in the top plate 54 for communication through atube 80 to the tube 40 of FIGURE l. The tube 8u provides a feedbackconnection which tends to maintain a pressure differential between theportion of the enclosure holding the water, and that inside the cup 62at a value which varies as a function of the gas flow rate.

Diffusion of the water through the porous walls of the cup 62 wets theinner surface thereof. The gas circulating through the cup picks up thewater to establish an equilibrium condition for the temperature at whichthe process operates. By wetting the gas in this manner, the amount ofoxygen picked up and held by the water is very small as compared to awetter where the gas is lbubbled directly through the water. Thus, thehold-up time of the system is very low.

To prevent the water from diffusing through the porous cup 62 at a ratewhic-h would cause water to collect inside the cup and block the gasline tubes, a portion of the cup is covered with a water barrier 82 oflucite or the like. Water diffuses through the porous material at a ratedetermined lby the difference in wetness between the inside and outsidesurfaces thereof. As a result of this feature, the area of the porousmaterial o-f the cup exposed to the water, and the average head of wateris selected in a noncritical fashion to provide substantially no excessoW of the water which would cause collection at the bottom of the cup62.

When the gas passing through the system picks up relatively largeamounts of the water and thereby tends to dry out the inside surface ofthe cup 62, the difference in wetness between the inside and outside oft-he cup increases the rate of diffusion of the water. When relativelylittle water is picked up by the gas stream, this differential wetnessis quite low thereby reducing the diffusion rate. The portion of the cup62 covered by the water barrier 82 provides a reservoir for any excesswater which may diffuse through the exposed portions of the porous cup62 and the system is self-stabilizing.

The improved oxygen analyzer cell 28 is shown in The top and bottomportions of the casing 84 are threaded to receive gas tube fittingsshown ln FIGURE 1. A porous silver cathode 86 is seated between a `boss88 formed in the casing 84 and a bushing 90 inserted into the end of thecasing. A Teflon plug 92 having a central a two piece clamp 96 fastenedabout the tube 26 (FIGURE l) is coupled `by a suitable fitting to admitVgas to the bottom of the casing 84.

A first anode electrode 98 comprising a strap of cadmium bent into aloop shaped configuration is positioned adjacent, but spaced from thecathode 86. The first anode 98, which is shown'in side view, provides asubstantial surface area in lconfrontation to the cathode. The firstanode is held in position by a screw 100 which passes through a Teflonplug 102 in the wall of the casing. The screw 100 is inserted through anaperture in the casing 84 blocked by a plug 104. The plugs 102 and 104are retained in position by a two piece clamp 106 which is fastenedabout the casing 84.

A second anode or impurity collector 108 comprises a strip of cadmiumextending to a position above the first 110 is inserted through anaperture in the casing 84 blocked by a plug 114. The plugs 112 and 114are retained in position -by a two piece clamp 116 which is fastenedabout the casing 84.

A third anode 118 comprises a stainless steel strip extending down intothe casing 84 about the same distance as the second anode 108. The thirdanode is supported by a screw 120 extending through a plug in the mannerdescribed above, and held in position by a clamp 122. Each of the plugsextending through the various apertures in the casing 84 have a lipportion which is pressed by the respective clamps against the body ofthe casing 84 to provide a watertight seal.

The casing 84 is filled with electrolyte, such as a solution`of KOH, toa level to contact all electrodes. As shown in the drawings, theelectrolyte extends to a level just above the clamp 106. The gas streamin passing through the analyzer cell tends to pick up the electrolyte inmist and molecular form. A porous Teflon filter 124 removes that portionof the electrolyte which is picked up in mist form, and that portionpicked up in molecular form is removed by the neutralizer 42 ofFIGURE 1. The electrolyte in mist form blocked by the filter 124 drainsback into the analyzer cell 28.

Electrical connections are made to the various electrodes by connectionto the screws 120, and 100 and the probe 94. The electrical circuit forthe analyzer is shown in FIGURE 6.

Power from an alternating current source is applied through atransformer to a bridge rectifier 132 to develop a direct voltage. Thedirect voltage is filtered by a capacitor 134 and a resistor 136, andapplied across a pair of series connected zener diodes 138 and 140. Aresistor 142 is connected in parallel with the series connected Zenerdiodes. The total voltage appearing across the diodes 138 and 140 isabout 1.5 volts.

The cathode of the diode 138 is connected to the third anode 118, andthe junction of the diodes 138 and 140 is connected to the first anode98. The anode of the diode 140, which is at the provide meter rangeshunts.

The current flowing through the oxygen analyzer cell is indicated on ameter 148. The meter 148 is driven by a low input impedance amplifier150, which may, by way of example, comprise a magnetic amplifier. Directvoltage for energizing the amplifier 150 is derived from a full settingof the range switch cient change in voltage drop across the resistor 144to affect the electrolysis action in the analyzer cell. Hence electrodesto reach a new equishort as the meter range switch 146 is moved fromposition to position.

The relatively inactive first anode 98 has a sufcient electrodepotentials are established.

The second anode 108 is at a potential of about `-0.75 volt relative tothe first anode 98 and the electrolyte. Similarly, the cathode is about0.65 volt relative to the first anode 98 and the electrolyte, that is0.75 volt less anode 118 is at about +0.75 volt positive relative to theiirst anode 98 and the electrolyte. These voltages are properly locatedwith respect to the known equilibrium potential to permit thequantitative oxygen-reduction process. At the same time the dissociationpotenti-al of water, approximately 1.7 volts, is

The electron current iiow between the cathode 86 and the third anode 118is a function of the amount of oxygen in the gas stream determined byFaradays Law of Electrolysis. The electron current flowing between theanode and cathode passes through the resistor extends the effectiveuseful life of this electrode.

144 and meter shunts therefor which are across the input terminals ofthe amplifier 150. Thus the magnitude of electron current determines thesignal input level to the amplifier 150 and the reading of the meter148.

impurities in the anodes and in the electrolyte solution producepositive ions. To prevent these ions from being attracted to the firstanode or cathode and coating it with the impurities, the second anodeelectrode 108 is provided with the most negative potential. In addition,since the electrode 108 is physically positioned much closer to theother anodes than is the cathode, any ions released by the anodes aremore strongly affected by the eld of the second anode 108, and areattracted thereto. The second anode 108, in reducing the coating of thefirst anode 98, As a result, the size of the anode 108 may be relativelysmall ascompared to corresponding anodes in known oxygen analyzer cellswhich provide an equivalent useful life. As a result the overall size,weight and cost of the oxygen analyzer all embodying the invention ismaterially reduced.

By virtue of the construction described, the oxygen analyzer cell of theinvention is considerably smaller, lighter and less expensixe than priorknown cells for efecting the same result` In addition the presentanalyzer cell has been found to be highly accurate and reliable inoperation, and well adapted for portable or semi-permanent applications.

What is claimed is:

Apparatus for the quantitative analysis of oxygen in a owing gas streamcomprising in combination of an electrolytic cell provided with a porouscathode element, an anode element in contact with an aqueous electrolytesolu-A tion, a gas sample supplying conduit opening adjacent saidcathode element to direct said flowing gas stream through said porouscathode element into intimate contact therewith and with saidelectrolyte solution to thereby effect electrolytic reduction of theoxygen in said flowing gas stream, a gas stream outlet port for saidelectrolytic cell, a wetter for humidifying said flowing gas stream tosubstantially an equilibrium condition for the temperature of saidelectrolyte prior to the time said gas stream is applied to said conduitopening, said wetter comprising an enclosure for an aqueous solution,means providing a porous container Within said enclosure, said porouscontainer sealed to prevent the entry therein of said solution except bydiffusion of the solution through the porous walls thereof, gas inletand outlet means into said container wherein gas admitted to saidcontainer through said gas inlet means is exposed to that portion ofsaid solution which has diffused to the inner Wall of said porouscontainer, means for coupling the gas outlet means of said wetter tosaid conduit opening, and a gas stream feedback connection communicatingbetween the gas stream outlet port for said electrolytic cell and theinterior of said enclosure.

References Cited by the Examiner UNITED STATES PATENTS 1,139,053 5/1915Murray et al 261-112 2,192,123 2/1940 Bennet 2041-195 2,255,069 9/1941Maier 55-16 2,508,238 5/l950 Pagen 204-195 2,757,132 7/1956 Northrop204195 2,857,979 10/1958 Van Dijck 55-318 2,876,189 3/1959 Spracklen etal 204-195 2,896,927 7/1959 Nagle et al. 261-112 2,898,282 8/1959 Flooket al. 204-195 2,924,630 2/1960 Fleck et al 55-16 2,943,036 6/1960Thayer et al 204-195 3,147,217 9/1964 Halton 261-122 3,236,759 2/1966Robinson 204-195 JOHN H. MACK, Primary Examiner. T. TUNG, AssistantExaminer.

